The present applicant has previously proposed a technique based on the above concept (Japanese Patent Application Laid-open No. 5-197423 corresponding to U.S. Pat. No. 5,350,912). In that technique, the vehicle is automatically driven exclusively by a steering assist system. On an actual road, however, a large number of insoluble problems are encountered in the automatic driving of the vehicle, e.g., when the lane change is conducted, as well as when the vehicle must swerve slightly to avoid an object which lies on the road, or when the vehicle must travel on the side of a lane, because pylons are arranged during repairing of the road. In fact, in such situations, it is required that the driver intervene in the operation of the steering assist system.
If the intervention of the driver is permitted, it is necessary to previously solve a problem of inequality between a steering angle determined by the control unit for the automatic traveling and a steering angle intended by the driver. This is because if the driver inputs a steering angle different from that in the steering assist system during operation of the steering assist system, the prior art system construed the steerage provided by the driver as an external disturbance, and an actuator works enough to realize the steering angle determined in the system. It follows that the system opposes the driver. Even if the system does not lead the opposition to the driver, if after a skirmish is once generated between the system and the person, the system suddenly stops the opposition, a disadvantage is encountered that the driver excessively steers to compensate more than necessary. If not only the system merely stops the opposition, but also urges the vehicle to travel along a new lane, there is generated a force permitting the vehicle to be dragged toward the new lane after the first opposition of the system. In this case, the excessive steerage is further considerable.
For example, if the lane change is carried out under the technique disclosed in the above Patent (Japanese Patent Application Laid-open No. 5-197423), an induction force which intends to bring the vehicle back to the original lane is applied after completion of the lane change. The magnitude of the induction force is considerably strong, because the vehicle is far away from the original lane. Thus, it is difficult to drive the vehicle along the new lane after the lane change.
A concept that the vehicle is automatically driven, for example, along each of a plurality of lanes, is disclosed in Japanese Patent Application Laid-open No. 6-255514. In this technique, an imaginary bank or incline is created in a process prepared for the lateral sides of each of the lanes, so that if the vehicle approaches a white line during traveling within the lane, a steering system functions to cause the vehicle to be brought back to the center of the lane as though the banked at the white lines on each side of each lane. In the technique disclosed in Japanese Patent Application Laid-open No. 6-255514, when the lane has been changed without operation of a turn signal blinker, it is required that the vehicle travel up the bank once and then down the bank. Apart from the problem of the force required for traveling up the bank in the existing lane, an opposite force permitting the vehicle to be brought into the new lane is applied during traveling down the bank. The driver is repulsed once and then, the repulsive force is suddenly changed to the bringing-in force. It follows that it is difficult for the driver to pass through the target course. This has a feeling as if during traveling on a road having tracks or prints for the wheels, the vehicle is shifted from one print for a wheel to another print for a wheel, and it is not possible for the driver to drive the vehicle as he desires.
In such an actual driving of the vehicle, the driver cannot afford the time to permit dissension with the system. During the dissension between the driver and the system, the lane change is high in priority, and the speed is also generally high and hence, the driver can little afford the time. Thus, an excellent man-machine interface is required and which is designed to permit the work to be shared between the system and the driver in carrying out the lane change.
For example, in the above Japanese Patent Application Laid-open No. 6-255514, it is possible to easily perform the traveling of the vehicle along the lane by means of the imaginary bank built in the process during a usual traveling. When the driver intends to try the lane change, however, if the driver's lane change intention can be perceived to decrease or eliminate the height of the imaginary bank made in the process, the lane change can be achieved easily as in the prior art.